1. Field of the Invention
The present invention relates to an electrically heated catalytic converter having a substrate for a catalyst formed as a laminated assembly of thin metal sheets.
2. Description of the Related Art
Exhaust gas purification devices that utilize three-way reducing and oxidizing catalytic converters disposed in the exhaust passage of an internal combustion engine are commonly used. Generally, the catalyst used in such converters purifies the pollutants in the exhaust gas only when the temperature of the catalyst exceeds a certain temperature, i.e., the catalysts in the converter do not work when the temperature of the catalyst is below an activating temperature.
Usually, once the engine starts, the catalyst in the converter is gradually heated by the exhaust gas of the engine and reaches the activating temperature. However, when the temperature of the engine is low, such as during a cold start, it takes a substantially amount of time to heat up the catalyst to the activating temperature, since the heat of the exhaust gas is absorbed by the cold walls of the exhaust passage before reaching the converter. Therefore, during a cold start of the engine, the exhaust gas from the engine is not sufficiently purified since the temperature of the catalyst is below the activating temperature.
To solve this problem, electrically heated catalytic converters are used to shorten the time required for the catalyst to reach the activating temperature. Usually, electrically heated catalytic converters have metal substrates, and the catalysts are heated during engine start by an electric current fed through the metal substrates, i.e., by using the metal substrates as electric heaters.
An electrically heated catalytic converter of this type, for example, is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-179939. The electrically heated catalytic converter disclosed in JPP'939 has a substrate formed as a scroll-like cylindrical laminated assembly of thin metal sheets.
More specifically, as shown in FIG. 16, the substrate in JPP'939 comprises a thin plain metal sheet 10 and a thin corrugated metal sheet 20 both provided with insulating coatings on the surfaces. The plain metal sheet 10 and the corrugated metal sheet 20 are placed one upon another and wound around a common center electrode 3 so that the plain metal sheet 10 and the corrugated metal sheet 20 form a scroll-like cylinder 2 of laminated metal sheets. The outer surface of the scroll-like laminated assembly 2 is connected to an outer electrode 5. In the layers 2a near the center electrode 3 and in the layers 2b near the outer electrode 5 of the laminated assembly, the crests of the corrugated metal sheet are soldered to the plain metal sheet in such a manner that electric currents can flow through the soldered parts. On the other hand, the corrugated metal sheet 20 and the plain metal sheet 10 are not soldered in the intermediate layers 2c between these soldered layers 2a and 2b, therefore, the plain metal sheet 10 and the corrugated metal sheet 20 are electrically isolated by the insulating coatings. Namely, in the substrate of the catalytic converter of JPP'939, conductive connections which connect the plain metal sheet and corrugated metal sheet of the respective layers are formed in the region 2a near the center electrode 3 and in the region 2b near the outer electrode 5, and between these regions, the plain metal sheets 10 and corrugated metal sheet 20 in the respective layers are electrically isolated each other.
After the scroll-like laminated assembly of the metal sheets is formed, a three-way reducing and oxidizing catalyst, of a known type, is attached to the surfaces of the thin metal sheets in each layer of the laminated assembly.
When an electric voltage is imposed between the electrodes 3 and 5, electric current flows in a radial direction in the region 2a near the center electrode and in the region 2b near the outer electrode through the conductive connections between the plain metal sheets 10 and the corrugated metal sheets 20. In the intermediate region 2c, the electric current flows only through the metal sheets, since the plain metal sheet 10 and the corrugated metal sheet 20 are electrically isolated by insulating coatings.
This means that in the regions 2a and 2b, radial electric paths having shorter lengths and larger cross sections are formed. On the other hand, a spiral electric path through the metal sheets (indicated by arrows in FIG. 16) which has a longer path length and smaller cross section is formed in the intermediate region 2c.
Therefore, when electricity is fed to the laminated assembly, the intermediate region 2c, which has a larger resistance than the regions 2a and 2b, generates a large amount of heat and reaches high temperatures. Thus, a spiral shaped electric heater of the thin metal sheets 10 and 20 is formed in the intermediate region 2c of the substrate.
When the intermediate region 2c of the substrate reaches the activating temperature (for example, 300.degree. to 400.degree. C.), an oxidation reaction, with the unburned HC and CO components in the exhaust gas, starts, and, once the reaction starts, the entire substrate is heated up rapidly by the heat generated by the oxidation reaction.
However, in the electrically heated catalytic converter in JPP'939, electric current flows uniformly through the metal sheets in the intermediate region. This means that, when electricity is fed to the converter, the entire volume of the cylinder formed by the metal sheets in the intermediate region 2c is uniformly heated by the electric current. Since the volume of the metal sheets in the intermediate region is relatively large, the total heat mass of the metal sheets in the intermediate region 2c is also relatively large. Therefore, a relatively long time is required to heat the metal sheets in this region up to the activating temperature of the catalyst.
In the electrically heated catalytic converter of JPP'939, it is possible to heat up the entire volume of the metal sheets in a short time by feeding a larger electric current. However, it is not practical to feed a large electric current to the converter during a starting operation of the engine since it increases the load on the battery and the alternator of the engine and may lead to a failure to start the engine or to wear of the battery.
To solve the above problems, electric paths connecting the center electrode and the outer electrode can be formed by local conductive connections between the thin metal sheets. In this case, small conductive connections of the respective layers are arranged in a predetermined pattern so that they form electric paths of narrow cross section which extend from the center electrode to the outer electrode in radial directions. Therefore, electric current flows through narrow electric paths formed by small conductive connections instead of flowing through the thin metal sheets. Since the electric current is concentrated in the small conductive connections instead of flowing through the cross section of the metal sheets uniformly, the conductive connections are heated rapidly without requiring a large amount of electricity.
When the conductive connections reach the activating temperature, the oxidation reaction, with the unburned HC and CO components, starts at the local conductive connections and, once the reaction starts, the heat generated by the oxidation reaction at the local conductive connections is conveyed to the rest of the substrate, and other portions also reach the activating temperature, thus the oxidation reaction starts in the entire substrate in a short time. Namely, when electricity is fed to the laminated assembly, the local conductive connections become heat spots which act as starters of the oxidation reaction in the catalytic converter.
Therefore, if the number of heat spots (i.e., the number of the local conductive connections) is large the time required for the catalytic converter to start the oxidation reaction becomes shorter. However, when the number of the local conductive connections are large, the total amount of electric current flowing through the local conductive connections also becomes large, and the failure in the starting operation of the engine and the wear of the battery may occur.
On the other hand, if the number of the local conductive connections are small, structural strength of the laminated assembly is lowered, since the thin metal sheets in the laminated assembly are joined only by the local conductive connections.
Therefore, it is difficult to optimize the number and size of the local conductive connections in such a manner that the oxidation reaction of the catalytic converter starts in a short time with relatively small amount of electric current while maintaining the structural strength of the laminated assembly.